Difficult to say. Have you tried undo?
That might work depending on how much you’ve done since.
Show New Screen is a bit different in that it is bound to an action, and the screen only exists while the action exists. So they are easy to lose if you aren’t careful. Which is all the more reason to use custom actions.
If the issue happened recently, try hitting the back/undo button in the Layout Editor (bottom left).
‘New Screen’ screens are a little tricky because they aren’t a tab and aren’t really attached to a table. It’s probably there somewhere in the ether, but not accessible to you. At least this is how I understand these ‘New Screen’ screens.